This invention relates Generally to a scroll compressor suited for use as a cooling medium compressor for freezing or air-conditioning purposes and a gas (e.g. air) compressor, and more particularly to a lubricating construction of such a compressor.
Japanese Patent Unexamined Publication No. 60-224988 discloses a lubricating construction of a scroll compressor including a stationary (fixed) scroll and an orbiting scroll, and a seal member provided at a rear side of the orbiting scroll to keep a bearing portion, provided at a central portion of the compressor, at a discharge pressure. In this construction, the interior of a sealed vessel is kept at the discharge pressure, and a crankshaft and bearing portions are disposed in an atmosphere of the discharge pressure, and the bearing portions are lubricated by a centrifugal pumping action caused by an eccentric hole formed in the crankshaft. Lubricating oil, after supplied to each bearing portion, is circulated under the discharge pressure within the sealed vessel.
In the conventional construction in which the lubricating oil is supplied to the bearing portions by the eccentric hole formed in the crankshaft, the lubricating oil is filled in the eccentric hole in the rotating crankshaft, thereby achieving the centrifugal pumping action. Therefore, this construction has been used only in scroll compressors of the vertical type in which the axis of the crankshaft is disposed vertically.
Generally, scroll compressors are of the vertical type in which a compression mechanism portion is disposed at an upper portion, and an electric motor portion is disposed at a lower portion. In order to obtain a pumping capacity of a centrifugal pump necessary for lubricating a bearing portion disposed at a upper portion of a crankshaft, it is necessary that a oil feed passage formed in the crankshaft be eccentric a predetermined amount from the axis of the crankshaft, and therefore it has been necessary to increase the outer diameter of the crankshaft in order to obtain the required centrifugal pumping capacity. Particularly when the scroll compressor is of the variable-speed type, there has been encountered a problem that a sufficient eccentricity amount capable of securing the necessary centrifugal pumping capacity during a low-speed operation can not be obtained because of the limited outer diameter of the crankshaft, thus limiting the low-speed operation range.
Japanese Patent Publication No. 62-37238 discloses another conventional construction in which gas pressure acts on a back pressure chamber at a rear side of an orbiting scroll to impart an axial urging force to the orbiting scroll, thereby preventing the orbiting scroll from being disengaged from a stationary scroll.
The back pressure chamber is sealed from a closed vessel by means of a frame, the crankshaft, a seal bearing mounted on the frame to support the crankshaft, and so on with respect to the pressure. Through a small hole formed in the orbiting scroll, the back pressure chamber communicates with a compression space in the process of being compressed, and therefore in accordance with a change in intake pressure, the back pressure chamber is kept to a pressure intermediate the suction pressure and the discharge pressure. Lubricating oil, supplied to the seal bearing, flows into the back pressure chamber, and then passes through the small hole in the orbiting scroll into the compression space to be mixed with the compressed gas there, and then is discharged to a space in a closed vessel. The lubricating oil is basically separated from the discharge gas in the closed vessel, but the lubricating oil not separated from the discharge gas is fed, together with the discharge gas, through a discharge pipe to a place where the gas is to be used. This phenomenon is called "oil shortage". Particularly when it is desired to design the compressor of a small size, a sufficient space for separating the oil from the gas can not be secured in the closed vessel, and therefore a sufficient separation can not be carried out, so that the amount of oil shortage increases.
The seal bearing seals the back pressure chamber with respect to pressure, and therefore one side of the seal bearing directed toward the space in the closed vessel is under the discharge pressure while the other side directed toward the back pressure chamber is at the pressure in the back pressure chamber (that is, the pressure intermediate the suction pressure and the discharge pressure; this pressure will hereinafter be referred to as "intermediate pressure"). Therefore, a pressure gradient from the discharge pressure to the intermediate pressure is produced in the heightwise direction of the seal bearing. When part of the lubricating oil, supplied to the seal bearing, flows into the back pressure chamber, the cooling medium dissolved in the lubricating oil produces bubbles due to the pressure reduction caused by this pressure gradient. When such pressure-reduction bubbles develop in the cooling medium, an oil film formed on the seal bearing breaks, so that the bearing can not perform the intended function, and therefore can not withstand a bearing load, which may results in seizure of the bearing.
The lubricating oil having flowed into the back pressure chamber flows through the small hole into the compression space in the course of compression to be mixed with the compressed gas therein. Since the lubricating oil of a temperature close to the discharge temperature is thus included in the gas in the course of compression, the temperature of the compressed gas rises. Namely, the compression work must be increased corresponding to this temperature rise, so that the compression efficiency is lowered. And besides, the lubricating oil mixed in the discharge gas must be separated therefrom in the closed vessel.